Lignin is a complex, high-molecular weight polymer occuring naturally in close association with cellulosic plants and trees. In the paper-making industry, lignin may be recovered as a by-product of the cellulose product by two principal wood-pulping processes known as the sulfite process and the kraft process. The sulfite process lignin is solubilized from the cellulosic portion of the wood pulp by direct sulfonation, while the kraft process is based on alkaline degradation mechanism causing cleavage of -aryl ether linkages in the polymeric lignin which sequentially result in chemical functions of the phenolic and carboxylic type. Kraft process lignin generally is isolated by acid precipitation of the black liquor of the pulping process at a pH below the pKa of the phenolic groups.
Lignin products are widely employed as additives in various chemical processes and compositions. Because of the high degree of chemical activity characteristic of lignin, it has been variously employed as a surfactant, extender, dispersant, reinforcement, solvent, binder, sequestering agent, emulsifier and emulsion stabilizer, and as a stabilizing and protecting colloid. Lignin is sold under the trademark INDULIN.RTM. by Westvaco Corporation of North Charleston, S.C.
Sulfonated lignin compounds, particularly sodium salts of lignosulfonates, are widely employed as additives and dispersants in textile dyestuffs and printing pigments, and have been sold for a number of years under the trademarks POLYFON.RTM. and REAX.RTM. by Westvaco Corporation. The degree of sulfonation of lignin generally determines its water solubility at various pH levels, e.g., sodium salts of sulfonated lignins are generally water soluble at alkaline and higher oH levels, while they are water insoluble at lower, or acid, pH levels.
My co-pending, commonly assigned U.S. patent application Ser. No. 07/298,677 filed Jan. 19, 1989 and Ser. No. 07/324,320 filed Mar. 16, 1989 describe the production of sulfonated lignin compositions having a high degree of sulfonation and water solubility by the reaction of lignin starting materials with sulfuric acid and oleum, respectively. In sulfonation of lignins, the sulfonic acid groups resulting from the sulfonation reaction attach directly to the aromatic ring of lignin in an ortho-position, relative to the principal lignin sidechain, as illustrated by the formula: ##STR1##
Sulfonation of lignins may also be carried out by a sulfomethylation process in which the lignin material is first methylolated with a suitable methylolating agent, such as formaldehyde, and thereafter treated with a sulfite or bisulfite salt, e.g., sodium bisulfite, to attach the sulfonic acid qroups to the methylol sidechain adjacent the phenol groups of the lignin material, as illustrated by the formula: ##STR2##
Sulfonation of lignins may also be carried out by sulfonation of the side chain of the lignin to produce a POLYFON.RTM. lignin product, as illustrated by the formula: ##STR3##
In U.S. Pat. No. 2,688,611, it has been proposed to produce lignin sulfates by treatment of lignin materials with a SO.sub.3 -group-containing compound selected from the group consisting of alkali metal chlorosulfonates, group II-A metal chlorosulfonates, and sulfur trioxide amine complexes in an anhydrous medium to produce sulfated lignins involving the hydroxyl groups of the phenol ring components of the lignin.
Highly sulfonated lignins are desirable for improved water solubility and in high electrolyte-containing applications, such as lignin additives in oil well drilling fluids, certain dyestuffs, ink formulations, micronutrients, concrete, and cement. Although salts negatively impact lignin solubility below the ionic charge requirements of such systems, solubility compensations can be made by increasing the sulfonic acid content of the lignin.
Solubility of the sulfated or sulfonated liqnin may be expressed in terms of its acid point, or acid precipitation point or number, which is the number of grams of a 50% concentration of sulfuric acid needed to start precipitation of a 35 milliliter solution of 0.5 weight percent lignin. As expressed, the higher the acid point of the lignin, the greater sulfonic acid/sulfate content and water solubility of the same.